![]() Such changes would modify the upper ocean circulation. ![]() The intense WBCs are typically associated with high levels of mesoscale activity that alter the characteristics of the WBCs for example WBCs tend to be faster, narrower and better located in eddy-permitting/resolving models compared to models that cannot resolve mesoscale activity 9.Ĭlimate models project consistent future changes to the surface winds 1 including a poleward expansion of the Hadley Circulation 10, a poleward intensification of the subtropical westerlies 11, and a slowdown of the Walker circulation and equatorial trade winds 12. Low-latitude WBCs (LLWBCs) also play a role in the leakage of water from the Pacific to Indian basins: the Mindanao Current (MC) and, to a lesser extent, the New Guinea Coastal Undercurrent (NGCU), feed the climatically important Indonesian Throughflow (ITF) 8. In the Indian and Pacific basins, however, some water from the Agulhas Current (AC) and East Australian Current (EAC) leak from the subtropical gyres, escaping westward south of Africa (AC extension) and Australia (Tasman Leakage), respectively. To conserve potential vorticity this flow is largely compensated by intense, poleward WBCs forming partially closed subtropical gyres. In the subtropics, north-south surface wind gradients, associated with climatological high-pressure systems, drive an equatorward Sverdrup transport in the ocean interior 7. The large-scale upper-ocean circulation is primarily driven by surface winds. Equatorward flowing WBCs at low latitudes play an important role in tropical circulation, facilitating cross-equatorial and inter-basin connectivity. Their poleward extensions form high-variability regions where the WBCs often leave the coast. ![]() Subtropical WBCs form the poleward flanks of the subtropical gyres. Particularly important for heat transport, air-sea interactions and marine ecosystems are the energetic Western Boundary Currents (WBCs) narrow (~100–200 km), intense (~1 m/s) jets in the upper several hundred meters off the continental shelves in the western basins. eggs, larvae), influencing the distribution of marine species 6. Surface ocean currents play a crucial role in the redistribution of heat 2, pollutants 3, plastics 4, biogeochemical tracers 5 and species dispersal-particularly during passive early-life stages (e.g. An often underappreciated aspect of climate change relates to upper ocean circulation. In moving to the latest model generation, despite structural model advancements, we find little systematic improvement in the simulation of ocean transports nor major differences in the projected changes.Īnthropogenic climate change manifests as increases in surface temperature and sea level, rainfall distribution changes and increasing frequency and intensity of certain extreme events 1. Intermodel differences in most future circulation changes can be explained in part by projected changes in the large-scale surface winds. For example, the East Australian Current, Brazil Current and Agulhas Current extensions are projected to intensify, while the Gulf Stream, Indonesian Throughflow and Agulhas Current are projected to weaken. Despite considerable differences in model structure, resolution and parameterisations, many currents show highly consistent projected changes across the models. ![]() Despite their coarse resolution, climate models successfully reproduce most large-scale circulation features with ensemble mean transports typically within the range of observational uncertainty, although there is often a large spread across the models and some currents are systematically too strong or weak. ![]() Using state-of-the-art climate models from the latest and previous Climate Model Intercomparison Projects, we evaluate upper ocean circulation and examine future projections, focusing on subtropical and low-latitude WBCs. They can affect regional climate and strongly influence the dispersion and distribution of marine species. Western Boundary Currents (WBCs) are important for the oceanic transport of heat, dissolved gases and nutrients. ![]()
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